With the advent of a deep reactive ion etching (DRIE) process for forming slots and trenches in a semiconductor substrate, greater precision and control over the etching of silicon substrates in higher speed processes has been obtained. DRIE is a dry etching process carried out under high vacuum by means of a chemically reactive plasma, wherein the constituents of the plasma are selected in congruence with the substrate to be acted upon. Before the adoption of DRIE techniques to form trenches or slots in semiconductor substrates, most trenches or slots in substrates greater than about 200 microns thick were formed by mechanical blasting techniques or chemical wet etching techniques. However, such mechanical techniques or chemical wet etching techniques are not suitable for newer products which demand higher tolerances and smaller trenches and/or slots. DRIE enables deep anisotropic etching of trenches and slots with greater tolerances and without regard to crystal orientation.
DRIE techniques have progressed incrementally towards a goal of etching high aspect ratio features in semiconductor substrates wherein the aspect ratio is on the order of 1:100 width to depth. Hence, much progress has been made in forming vertical conduits or trenches with substantially perpendicular walls. The process scheme for achieving high aspect ratio slots or trenches in semiconductor substrates includes a series of sequential steps of alternating etching and passivation. Such aniosotropic etching techniques are described in U.S. Pat. Nos. 5,611,888 and 5,626,716 to Bosch et al. the disclosures of which are incorporated herein by reference.
A schematic diagram of a DRIE system 10 is illustrated in FIG. 1. The system 10 includes a ceramic reaction chamber 12 and a radio frequency (rf) unit 14 for providing source power to a coil 16 to generate a plasma in the reaction chamber 12. A wafer 18 containing a plurality of semiconductor substrates is disposed in the chamber 12 on a cooled chuck which is part of platen 20. The temperature of the platen/chuck 20, and thus the wafer 18, is selected on a chiller unit 22 providing helium gas to the platen/chuck 20. A platen power unit 24 provides rf biasing power to the platen 20 during the etching process. The chamber 12 is maintained at a low pressure during etching by a vacuum pumping unit coupled to a vacuum port 26. A reactive gas is introduced into the chamber through a gas inlet port 28. A bellows system 30 may be provided to adjust a height of the platen 20 before the etching process.
Accordingly, most dry etching systems 10 are designed to etch substantially vertical wall slots and trenches in the substrate 18, i.e., walls that are substantially perpendicular to a surface of the substrate 18. However, for micro-fluid ejection heads, it has been found that substantially vertical walls may entrap more air in fluids passing through relatively narrow slots. Such air entrapment can lead to fluid starvation for ejection devices on a device surface of the substrate. Accordingly, there is a need for improved DRIE techniques to form fluid feed slots having reentrant walls in micro-fluid ejection head substrates.
With regard to the foregoing, there is provided a method of micro-machining a semiconductor substrate to form through slots therein and substrates made by the method. The method includes providing a dry etching chamber having a platen for holding a semiconductor substrate. During an etching cycle of a dry etch process for the semiconductor substrate, a source power is decreased, a chamber pressure is decreased from a first pressure to a second pressure, and a platen power is increased from a first power to a second power. Through slots in the substrate provided by the method have a reentrant profile for fluid flow therethrough.
In another embodiment there is provided a deep reactive ion etching process for etching a semiconductor substrate to form one or more reentrant fluid feed slots therein. The process includes decreasing a source power from during etching cycle steps of the etching process, decreasing a chamber pressure from a first pressure to a second pressure during etching cycle steps of the etching process, and increasing a platen power from a first power to a second power during etching cycle steps of the process.
An advantage of the exemplary process disclosed herein can include providing precisely formed slots having a reentrant profile without significantly reducing a production rate for micro-machining semiconductor substrates. For example, production rates may be maintained by ramping the powers and pressure during the etching cycles of the process rather than maintaining constant powers and pressure throughout the process. The exemplary process can also enable the formation of slots having reentrant profiles with reduced top side damage. Despite a reduction in chamber pressure and a decrease in source power during the etching cycles of the process, the process can yield superior reentrant slot profiles, which is believed to be contrary to conventional thinking with regard to such processes.